1. Field of the Invention
The present invention is related generally to the field of solid state light emitting diode and laser fabrication, and more particularly to techniques for controlling the structure and surface qualities of layers of such structures for improved device performance, reliability, and lifespan.
2. Description of the Prior Art
In the III-V compound semiconductor family, the nitrides have been used to fabricate visible wavelength light emitting diode and laser active regions. They also exhibit a sufficiently high bandgap to produce devices capable of emitting light in the ultraviolet, for example wavelengths between 300 and 400 nanometers. In particular, InAlGaN systems have been developed and implemented in visible and UV spectrum light emitting diodes (LEDs), such as disclosed in U.S. Pat. No. 6,875,627 Bour et al., which is incorporated herein by reference. These devices are typically formed on an Al2O3 (sapphire) substrate, and comprise thereover a GaN:Si or AlGaN template layer, an AlGaN:Si/GaN supperlattice structure for reducing optical leakage, an n-type electrode contact layer, a GaN n-type waveguide, an InGaN quantum well heterostructure active region, and a GaN p-type waveguide region. In addition, the complete device may also have deposited thereover a p-type AlGaN:Mg cladding layer and a capping layer below a p-type electrode.
While significant improvements have been made in device reliability, optical power output, and mode stability, the performance of the nitride-based light emitting diode emitting in the ultraviolet (UV) is still far inferior to that of blue or green light emitting devices. It is particularly true that for the UV light emitting devices, the nature of the substrate and template layer have a critical impact on the overall device performance. For example, electrical resistance between the structural layers of the device significantly effects optical output. While Al2O3 (sapphire) as a substrate has numerous advantages, the AlGaN template layer formed over the typical Al2O3 substrate posses high series resistance due to limited doping capabilities. Furthermore, the crystallographic structure of the device layers plays a key role in the device's operational characteristics, and the AlGan template layer provides a relatively poor crystalline template.
The dislocation densities in AlGaN or AlN template layers on sapphire are typically in the mid 109 to high 1010 cm−2 range. As a consequence, the external quantum efficiencies of deep UV light emitting diodes in the 250 nm to 350 nm range are still below 2% even for the very best devices (external quantum efficiencies near 50% have been demonstrated for blue GaN-based LED structures). The high dislocation densities in AlGaN or AlN template layers on sapphire also pose significant problems for the light emitting diode device lifetimes.
GaN epitaxial layers on sapphire substrates have proven to be a preferred template for InAlGaN film growth, providing excellent optoelectronic quality for visible light emitting diode devices and reasonable dislocation densities. The dislocation densities in GaN template layers on sapphire are typically in the low 109 to mid 107 cm−2 ranges. Accordingly, sapphire with a GaN template layer is the preferred foundation for visible GaN-based light emitting diodes.
However, while forming an excellent growth substrate, the GaN/sapphire system poses significant problems from the perspective of finished device performance for UV LEDs. One issue is the large lattice mismatch between the GaN buffer layer and the high aluminum content AlGaN layers that are necessary for UV LEDs (e.g. the aluminum content in the multiple quantum well active region of a 280 nm LED is as high as 50%). The UV InAlGaN heterostructure grown on GaN/sapphire are under tensile stress, which causes cracking of the AlGaN epitaxial layers when the critical layer thickness is exceeded. The critical thickness for an AlGaN film with 50% aluminum mole fraction is about 20-50 nm, which is much to thin for realizing a usable device structure. Accordingly, methods and structures such as the superlattice disclosed in the provisional U.S. patent application Ser. No. 60/736,362, titled “Superlattice Strain Relief Layer For Semiconductor Devices”, referred to and incorporated by reference above, have been developed to allow increase Al content while reducing or eliminating strain induced cracking.
However, two additional problems remain. First, the sapphire substrate is electrically insulating. Therefore, for LED devices grown on sapphire substrates, all contacts must be made from the topside of the device (opposite the growth substrate). This complicates the contacting and packaging scheme and also leads to an increase in operating voltage due to the resistance of the n-AlGaN current spreading layer. This is particularly acute for deep UV emitters, as the electron mobilities decrease with increasing aluminum mole fraction, and the maximum Si-doping levels are low in AlGaN films. And second, the GaN layer is absorptive at UV wavelengths. While architecturally the goal is to provide a deep UV laser which emits from its bottom side, the absorption by the GaN layer precludes producing sufficiently high optical output.
The problems imposed by the sapphire structure have been significantly overcome by methods for removal of the substrate from the completed structure. One such method is referred to herein as a laser lift-off (LLO) process, for example as described in U.S. Pat. No. 6,757,314, which is incorporated by reference herein. One embodiment of an LLO method bonds a combination substrate/heat sink to the device topside, the surface opposite the sapphire growth substrate. A nano-pulsed excimer laser, whose energy is absorbed at the sapphire/GaN interface, is employed to rapidly heat then cool a microlayer at the sapphire/GaN interface, which decouples the substrate from the GaN layer, allowing removal of the substrate. A variation of this method first bonds an intermediate wafer to the device topside. LLO then allows for removal of the substrate, bonding the device to a heat sink/electrically conductive substrate at the surface previously occupied by the sapphire substrate, and finally removal of the device from the intermediate wafer. In such a structure, electrical contact may be made directly to the underside of the device.
However, we have discovered that a consequence of the LLO process is that the exposed surface of the GaN layer is left rough, uneven, and its plane out of parallel to the plane of the other layer interfaces. When etching, this surface morphology and orientation is translated directly to the underlying layers as the anisotropic etch proceeds. The uniform etch rate of the chemical process which removes the GaN layer means that valleys and hillocks created in the GaN layer ultimately become valleys and hillocks in the heterostructure active region where, according to one embodiment, the chemical removal process is designed to cease. These valleys and hillocks can be of significant size when compared to the thickness of the layers comprising the heterostructure active region, and indeed can render the heterostructure active region inoperable. Stopping the etch above the heterostructure active region is difficult, and even where possible, if the layer above the heterostructure active region is comprised of GaN, such as in the case where a GaN/AlN superlattice structure is employed to reduce strain in high Al layers, the remaining GaN will absorb the UV emission and effect device performance. A similar set of consequences stem from a surface plane of the GaN being out or parallel with the surface planes of the underlying layers.
Accordingly, there is a need in the art for a method of producing a deep UV light emitting device. Specifically, there is a need for a method of providing an improved surface upon completion of the removal of the substrate and etching of selected layers for a deep UV light emitting device. Removal of the substrate and GaN layers must be accomplished while also providing a suitable starting point for anisotropic etching. The method must be compatible with the general processing requirements for AlGalnN LED and laser devices, and should not significantly increase the cost or complexity of manufacturing such devices. Finally, the method must permit the formation of additional structure for, and support the ultimate formation of a LED or laser which emits light from the surface at which the growth substrate (e.g., sapphire) was initially secured.